This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-333781, filed Oct. 31, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydrodynamic bearing device composed of a shaft and a sleeve for rotatably supporting this shaft, wherein a dynamic pressure generating groove is provided on a surface of the shaft or sleeve. More particularly, the present invention relates to a means for preventing leakage of a fluid (for example, lubricant) in a hydrodynamic bearing.
2. Description of the Related Art
In general, a hydrodynamic bearing is composed of a shaft, a sleeve for rotatably supporting the shaft, a thrust receiver fixed to a lower face of the sleeve, and a lubricant filled in the sleeve. When the shaft is rotated, a dynamic pressure is generated with the lubricant in a radial direction due to an action of a herringbone groove provided on a wall face of the inner periphery of the sleeve. Then, the shaft rotates in non-contact with the wall face of the inner periphery of the sleeve. In a thrust direction, the shaft rotates while a tip end of the shaft comes into point contact with a thrust receiver via a lubricant, and frictional energy loss is minimized.
However, there is a problem that the lubricant tends to move, by a variety of factors, towards a sleeve opening end, (hereinafter referred to as xe2x80x9cmovement forcexe2x80x9d) and the leakage of the lubricant from the wall face of the inner periphery of the sleeve occurs. Some of the above factors are as follows.
1. The capillary phenomenon occurs in a gap between a shaft and a sleeve, which shifts the lubricant vertically (in a vertical direction), and the movement force occurs.
2. A lubricant rotates with shaft rotation, and the lubricant to which centrifugal force is applied is subjected to a restriction on the wall face of the inner periphery of the sleeve. Thus, the vertical movement force occurs.
3. A lubricant is swelled due to rotational heat generation, and the vertical movement force acts on a lubricant.
4. A pressure difference occurs inside a bearing due to inaccuracies in the precision of a herringbone groove, and an upward or downward movement force acts on a lubricant. A herringbone groove free of generating the pressure difference requires high precision.
In the case where the movement of the lubricant caused by any of these factors cannot be restrained, a lubricant leakage occurs from the sleeve opening end. Then, the lubricant in a gap between the shaft and the wall face of the inner periphery of the sleeve decreases, the shaft and sleeve come into contact with each other, and an oscillation noise is increased. As a result, the service life of the bearing itself is significantly reduced. Therefore, how well the hydrodynamic bearing prevents the lowering of performance or reliability caused by the leakage of the lubricant is an important problem in the hydrodynamic bearing.
Conventionally, a means for preventing the leakage of a lubricant of such a hydrodynamic bearing is proposed as follows. For example, a dynamic pressure type hydrodynamic bearing described in Japanese Patent No. 2574666 comprises a shaft, a sleeve for rotatably supporting the shaft, a thrust receiver fixed to the lower face of the sleeve, and a lubricant filled in the sleeve, wherein a herringbone groove is formed at a proper site in the thrust direction on the sleeve inner periphery wall face.
This herringbone groove in the vicinity of a sleeve opening end is asymmetrical in a thrust direction, and is short in distance at a side proximal to the sleeve opening end. In addition, in the vicinity of the sleeve opening end, a tapered peripheral groove is provided at the shaft so that the axial diameter is shorter at the portions of the shaft distant from the herringbone groove.
With the above described construction and features, in the prior art, when the shaft is rotated, a return force, towards the inside of the bearing, acts due to a pressure difference in the asymmetrical herringbone groove in the vicinity of the sleeve opening end, thereby preventing the leakage of the lubricant. In addition, this return force acts on the lubricant at the tapered portion of the shaft so that leakage of the lubricant can be further prevented.
However, in recent years, a disk recording/reproducing apparatus is thinner, and is higher in density, a motor for use in the recording/reproducing apparatus becomes thinner, and the oscillation precision of the shaft that is a center of disk rotation requires higher precision. Due to this thinning, the lengths of the motor shaft and sleeve are reduced. Thus, if an effective span of a radial bearing is determined so as not to impair the oscillation precision of this shaft, it becomes difficult to ensure a space for a means for preventing the leakage or evaporation of the lubricant in a dynamic bearing device and a space for holding the lubricant in the bearing.
In a conventional means for preventing the leakage or evaporation of the lubricant in a dynamic pressure bearing, a space for a length in the thrust direction of the substantially tapered portion provided at a part of the shaft cannot be provided, thus making it impossible to push back the lubricant sufficiently. Thus, the advantageous effect cannot be attained. Alternatively, in the case where the above length is defined such that the advantageous effect of the substantially tapered portion can be attained, the effective span of the radial bearing is reduced. Thus, there occurs a failure that a shaft oscillation range is increased.
In addition, in the case where the herringbone groove in the vertical direction of the bearing is made asymmetrical, preventing leakage or the like, the asymmetrical degree of such a herringbone groove cannot be sufficiently obtained. Thus, the advantageous effect cannot be attained. Alternatively, in order to attain the advantageous effect with a short asymmetrical degree, high precision is required in groove processing. Thus, there is a problem that higher costs are inevitable.
It is an object of the present invention to provide a dynamic pressure bearing device (hydrodynamic bearing device) capable of sufficiently preventing the evaluation and leakage of the lubricant in order to improve the reliability of a hydrodynamic bearing for a thin motor.
In order to solve the foregoing problem and achieve the above object, the present invention takes the following means. That is, according to a first aspect of the present invention, there is proposed a hydrodynamic bearing device having: a shaft; a sleeve rotatably supporting the shaft, wherein a herringbone groove is formed at a predetermined part of an inner periphery wall face of the sleeve, and a thrust receiver supporting a thrust side of the shaft fixed to a lower face of the sleeve, thereby sealing predetermined lubricant; wherein a spiral groove which is narrower in groove width and shallower in depth than the herringbone groove, and which is oriented to the thrust receiver in the rotation direction of the shaft, is formed on the inner periphery wall face of the sleeve. Then, the force along the first spiral groove acts on the lubricant in a sealed space due to a pumping action of the first spiral groove, and the lubricant is oriented to the thrust receiver via the herringbone groove.
In addition, the herringbone groove and the first spiral groove is a groove that is formed so as to be superimposed on the wall face of the inner periphery of the sleeve.
The above first spiral groove is a groove that is formed over the entire wall face of the inner periphery of the sleeve.
There is proposed the previously described hydrodynamic bearing device, wherein a second spiral groove having a narrower groove width and a shallower depth than the herringbone groove, the second spiral groove being oriented in the sleeve opening end direction in the rotation direction of the shaft, is further formed on the wall face of the inner periphery of the sleeve, and wherein the movement force of the lubricant toward the thrust receiver due to the first spiral groove can be restrained by the second spiral groove.
In addition, there is proposed the previously described hydrodynamic bearing device, wherein, in the sleeve, a large diameter peripheral groove is formed in the vicinity of the opening end, and in the inner diameter of the sleeve, the inner diameter of the opening end side close to the large diameter peripheral groove is greater than that of the thrust receiver side. The large diameter peripheral groove maintains swelling of the lubricant due to a temperature change.
Still furthermore, there is proposed the previously described hydrodynamic bearing device, wherein, in the first spiral groove, the groove at the opening end side is constructed to be wider in groove width and deeper in depth than that at the thrust receiver side close to the large diameter peripheral groove. The spiral groove at the opening end side is formed wide and deep, whereby a gap between the shaft and the sleeve is further increased. The capillary phenomenon can be greatly restrained, and a lubricant returns to the large diameter peripheral groove via this spiral groove.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.